Observation of Topological Band Gap Solitons

TL;DR

This paper reports the experimental discovery of topological band gap solitons in a photonic Floquet topological insulator, revealing unique cyclotron-like orbits due to the lattice's topology and nonlinearity.

Contribution

It demonstrates the existence of topological solitons in a photonic system, bridging topological physics with nonlinear wave phenomena in a novel experimental setup.

Findings

Observation of topological solitons in a photonic lattice

Solitons exhibit cyclotron-like orbits linked to lattice topology

Nonlinearity arises from optical Kerr effect in the system

Abstract

Topological materials exhibit properties dictated by quantised invariants that make them robust against perturbations. This topological protection is a universal wave phenomenon that applies not only in the context of electrons in solid-state materials but also to photonic systems, ultracold atoms, mechanical systems, circuits, exciton-polaritons and beyond. However, the vast majority of research in these systems has focused on the linear domain, i.e., where inter-particle interactions do not play a role. Here, we experimentally observe solitons -- waves that propagate without changing shape as a result of nonlinearity -- in the bulk of a photonic Floquet topological insulator. These solitons exhibit fundamentally different behaviour than solitons in ordinary band gaps in that they execute cyclotron-like orbits that are associated with the topology of the lattice. Specifically, we employ a laser-written waveguide array with periodic variations along the waveguide axis that give rise to non-zero Floquet winding number, where the nonlinearity arises from the optical Kerr effect of the ambient glass. The effect described here is applicable to a range of bosonic systems due to its description by the focusing nonlinear Schr\"odinger equation, i.e., the Gross-Pitaevskii equation with attractive interactions.